A control unit of the brake apparatus for the vehicle disclosed in JP2006-21745A generates a braking force corresponding to a target braking force by means of a cooperative braking control operated using a hydraulic braking force generated by a hydraulic brake apparatus together with a regenerative braking force generated by a regenerative brake apparatus. Specifically, the known control unit of the brake apparatus for the vehicle includes a hydraulic brake apparatus, a regenerative brake apparatus and a braking force compensating means. The hydraulic brake apparatus generates, independently from a braking operation of a driver, a hydraulic braking force for each wheel by applying a brake fluid pressure generated by driving a pump to a wheel cylinder provided at each wheel.
The regenerative brake apparatus generates a regenerative braking force by driving the motor generator. The regenerative braking force corresponds to a braking operation state detected by a brake operation detecting means for detecting the braking operation.
The braking force compensating means compensates a shortage of the braking force, which is caused by variations of the regenerative braking force.
According to the known control unit of the brake apparatus for the vehicle, a driver's request of the braking force is determined on the basis of a master cylinder hydraulic pressure, which varies depending on a level of a depression of a brake pedal.
In this configuration, a target braking force is set on the basis of the variation of the master cylinder hydraulic pressure, and in order to obtain a braking force corresponding to the target braking force, the cooperative braking control using the hydraulic braking force generated by the hydraulic brake apparatus and the regenerative braking force generated by the regenerative braking force.
Further, when the regenerative braking force is reduced, the reduced amount is replaced with the hydraulic braking force. In this situation, a pump provided at the hydraulic brake apparatus sucks the brake fluid from the master cylinder, and such sucked brake fluid is supplied to the wheel cylinder, as a result, an appropriate hydraulic braking force is generated.
However, because the known brake apparatus employs a so called in-line system, in which the master cylinder is connected to the wheel cylinder, when the brake fluid in the master cylinder is sucked by the pump, a level of the master cylinder hydraulic pressure will be reduced. In this situation, because the target braking force is set on the basis of the master cylinder hydraulic pressure, the target braking force may be set to a lower value. In order to overcome such inconvenience, a pressure difference control valve or the like for generating a pressure difference may be provided between the master cylinder and the wheel cylinder in order to control the wheel cylinder hydraulic pressure so as to be higher than the master cylinder hydraulic pressure. In consideration of the pressure difference generated at the pressure difference control valve, the wheel cylinder hydraulic pressure is set and applied to each wheel.
In this configuration, the pressure difference may vary depending on the target braking force, which varies according to the reduction of the master cylinder hydraulic pressure, as a result, a pressure difference output value (control electric current) generated at the pressure difference control valve is reduced. In this situation, the wheel cylinder hydraulic pressure is temporally reduced; as a result, the driver may have a poor brake feeling.
A need thus exists to provide a control unit of a brake apparatus for a vehicle by which the poor brake feeling caused by suction of a brake fluid in the master cylinder is be improved.
Furthermore, a brake apparatus for a vehicle automatically controls hydraulic pressure of wheel cylinders independently from an operation of a brake-operating member such as a brake pedal by a driver has conventionally been known. For example, an automatic braking device for a vehicle disclosed in JP2004-9914A includes: a master cylinder generating a basic hydraulic pressure (master cylinder hydraulic pressure) based upon an operation of a vacuum booster according to the brake-pedal operation by a driver; a pump generating a pressurizing hydraulic pressure so as to be higher than the basic hydraulic pressure by sucking the brake fluid at a hydraulic pressure circuit (at a discharging side of the master cylinder) having the basic hydraulic pressure; and two normally open linear solenoid valves by which the amount of pressurization (pressure differences) for respective systems to be applied to the basic hydraulic pressure is controlled using the pressurizing hydraulic pressure generated by the pump.
For example, when a distance between a vehicle to which the above-described device is mounted and another vehicle driving ahead falls below a predetermined reference distance, this automatic braking device controls the pump and the linear solenoid valve so as to adjust the amount of pressurization. Here, a wheel cylinder pressure is generated by adding the amount of pressurization to the basic hydraulic pressure. The hydraulic braking force is automatically controlled by applying such wheel cylinder pressure to the corresponding wheel cylinder. As a result, the vehicle is automatically applied with braking force independently of the operation of the brake-pedal operation of the driver.
As disclosed in JP2006-21745A, recent developments have led to a technology in which a regenerative cooperative braking control is implemented, which uses hydraulic braking force and uses a motor's regenerative braking force. In this case, the above-described automatic braking device is applied to vehicles, which use motors as power supply, or to what-is-called hybrid-electric vehicles (HEV), which use internal combustion engines and electric batteries to power electric motors.
More specifically, in the brake device disclosed in JP2006-21745A, the boosting characteristics of a vacuum booster is determined so that a basic hydraulic pressure corresponding to a depression amount against a brake pedal becomes lower than a predetermined target pressure value by a specified amount. As a result, “hydraulic braking force (basic hydraulic pressure braking force) based upon the basic hydraulic pressure (master cylinder hydraulic pressure) can be lower than a preset target value by a specified amount.
Here, a compensation braking force is a regenerative braking force by a motor, a hydraulic braking force based upon the amounts of pressurization, or a combination thereof. A total braking force is the sum of the basic hydraulic pressure braking force and the compensation braking force. The compensation braking force, i.e., a regenerative braking force and/or a pressurizing hydraulic braking force, is adjusted according to the depression amount against the brake pedal. As a result, characteristics of the total braking force relative to the depression amount of the brake pedal then agrees with predetermined target characteristics, thereby preventing the driver from having an uncomfortable feeling when braking.
Further, it has been recently proposed that a vacuum booster, having the aforementioned boosting characteristics, is applied to a vehicle, which employs only an internal combustion engine as power supply and is applied with the above-described automatic braking device. In such cases, a compensation braking force consists of only a pressurizing hydraulic braking force. That is, a total braking force is the sum of the basic hydraulic pressure braking force and the compensation braking force having only the pressurizing hydraulic braking force. The compensation braking force (pressurizing hydraulic braking force) is adjusted corresponding to the depression amount of the brake pedal in a manner that characteristics of the total braking force relative to the depression amount against the brake pedal agree with preset target characteristics.
Accordingly, it is possible to use a small-sized vacuum booster so that it can be readily mounted on a vehicle. Further, characteristics of the total braking force relative to the depression amount of the brake pedal is designed to have more flexibilities so that various effects can be expected.
According to the automatic braking device in the JP2004-9914, the pressure circuit employs a configuration (hereinafter referred to as an in-line system) in which the pump sucks the brake fluid from the hydraulic pressure circuit (outlet port of the master cylinder) at which basic hydraulic pressure (master cylinder hydraulic pressure) is generated. However, in this configuration, even when the driver intends to sustain the brake pedal operation to a certain level, the basic hydraulic pressure may vary.
Because the target braking force (or the target wheel cylinder hydraulic pressure), which is determined on the basis of such basic hydraulic pressure, varies in accordance with the variation of the basic hydraulic pressure, the variation of the basic hydraulic pressure continues. This continuation of variation is referred to as “basic hydraulic pressure hunting”.
When the basic hydraulic pressure hunting occurs, vibration caused by the hunting is transmitted to the brake pedal, and the driver may have poor brake feeling. A reduction of such basic hydraulic pressure hunting has been expected so far.
Another need thus exists to provide a control unit of a brake apparatus for a vehicle reducing basic hydraulic pressure hunting. Specifically, such control unit is applied to the braking apparatus employing an in-line system, and the braking apparatus generates a total braking force by adding the compensation braking force to the basic hydraulic braking force (=basic hydraulic braking force+compensation braking force).